Google has released an easy to use API that creates charts and
lets you very easily imbed them in your web pages. For example,
here's a chart that shows recent browser usage for visitors to this site:

The code that produces that is as easy as you could want: I simply
put "http://chart.apis.google.com/chart?cht=p3&chd=t:45.8,38.7,15.5&chs=270x100&chl=Firefox|IE|All%20Others" inside an <img> tag.

I first got wind of this from a post at Technometria, by the way. Good
site, worth a visit.
I could have made it a line chart just by changing the "cht=p3" to "cht="lc":

Fantastic stuff, and pretty easy to use. However, I ran into trouble
when I wanted to make a bar chart. Let's try it:

Ugh. That's kind of cramped. How do we spread it out? Maybe it just needs more room, let's try making it wider:

No, that didn't do it. We need to add a new parameter: "chbh=40,20". That
says the bars are 40 pixels wide, with 20 pixels between them.

There, that's better. The data provided in these examples has to
be in the range of 0 to 100 (and -1 for missing data), but Google
provides two other ways to encode larger values and even a little bit
of Javascript so that you don't have to do the encoding yourself. Take
a look at the Developer's Guide page for all the details.

Note the usage policy:

Use of the Google Chart API is subject to a query limit of 50,000
queries per user per day.